There are also 'bidirectional' shift registers, which allow shifting in both directions: L → R or R → L. There are also types that have both serial and parallel input and types with serial and parallel output. These are often configured as 'serial-in, parallel-out' (SIPO) or as 'parallel-in, serial-out' (PISO). Shift registers can have both parallel and serial inputs and outputs. There is an inherent trade-off in the design of bit arrays putting more flip-flops in a row allows a single shifter to store more bits, but requires more clock cycles to push the data through all of the shifters before the data can be read back out again. Data was stored into the array and read back out in parallel, often as a computer word, while each bit was stored serially in the shift registers.
In most cases, several parallel shift registers would be used to build a larger memory pool known as a ' bit array'. By connecting the last flip-flop back to the first, the data can cycle within the shifters for extended periods, and in this configuration they were used as computer memory, displacing delay-line memory systems in the late 1960s and early 1970s. They share a single clock signal, which causes the data stored in the system to shift from one location to the next.
Computer memory unit using cascaded flip-flopsĪ shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next.
